Reducing location-dependent destructive interference in
distributed antenna systems (dass) operating in multiple-input,
multiple-output (mimo) configuration, and related components,
systems, and methods

ABSTRACT

Components, systems, and methods for reducing location-dependent destructive interference in distributed antenna systems operating in multiple-input, multiple-output (MIMO) configuration are disclosed. Interference is defined as issues with received MIMO communications signals that can cause a MIMO algorithm to not be able to solve a channel matrix for MIMO communications signals received by MIMO receivers in client devices. These issues may be caused by lack of separation (i.e., phase, amplitude) in the received MIMO communications signals. Thus, to provide amplitude separation of received MIMO communications signals, multiple MIMO transmitters are each configured to employ multiple transmitter antennas, which are each configured to transmit in different polarization states. In certain embodiments, one of the MIMO communications signals is amplitude adjusted in one of the polarization states to provide amplitude separation between received MIMO communications signals. In other embodiments, multiple transmitter antennas in a MIMO transmitter can be offset to provide amplitude separation.

PRIORITY APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/447,014, filed on Jul. 30, 2014, the content of which is relied uponand incorporated herein by reference in its entirety and the benefit ofpriority under 35 U.S.C. §120 is hereby claimed.

BACKGROUND

The disclosure relates generally to distribution of data (e.g., digitaldata services and radio-frequency communications services) in adistributed antenna system (DAS) and more particularly tomultiple-input, multiple-output MIMO technology, which may be used inthe DAS.

Wireless customers are demanding digital data services, such asstreaming video signals. Concurrently, some wireless customers use theirwireless devices in areas that are poorly served by conventionalcellular networks, such as inside certain buildings or areas where thereis little cellular coverage. One response to the intersection of thesetwo concerns has been the use of distributed antenna systems.Distributed antenna systems can be particularly useful to be deployedinside buildings or other indoor environments where client devices maynot otherwise be able to effectively receive radio-frequency (RF)signals from a source. Distributed antenna systems include remote units(also referred to as “remote antenna units”) configured to receive andwirelessly transmit wireless communications signals to client devices inantenna range of the remote units. Such distributed antenna systems mayuse Wireless Fidelity (WiFi) or wireless local area networks (WLANs), asexamples, to provide digital data services.

Distributed antenna systems may employ optical fiber to supportdistribution of high bandwidth data (e.g., video data) with low loss.Even so, WiFi and WLAN-based technology may not be able to providesufficient bandwidth for expected demand, especially as HD video becomesmore prevalent. WiFi was initially limited in data rate transfer to12.24 Mb/s and is provided at data transfer rates of up to 54 Mb/s usingWLAN frequencies of 2.4 GHz and 5.8 GHz. While interesting for manyapplications, WiFi bandwidth may be too small to support real timedownloading of uncompressed HD television signals to wireless clientdevices.

MIMO technology can be employed in distributed antenna systems toincrease the bandwidth up to twice the nominal bandwidth, as anon-limiting example. MIMO is the use of multiple antennas at both atransmitter and receiver to increase data throughput and link rangewithout additional bandwidth or increased transmit power. However, evendoubling bandwidth alone may not be enough to support high bandwidthdata to wireless client devices, such as the example of real timedownloading of uncompressed high definition (HD) television signals.

The frequency of wireless communications signals could also be increasedin a MIMO distributed antenna system to provide larger channel bandwidthas a non-limiting example. For example, an extremely high frequency(EHF) in the range of approximately 30 GHz to approximately 300 GHzcould be employed. For example, the sixty GHz (60 GHz) spectrum is anEHF that is an unlicensed spectrum by the Federal CommunicationsCommission (FCC). EHFs could be employed to provide for larger channelbandwidths. However, higher frequency wireless signals are more easilyattenuated and/or blocked from traveling through walls, buildingstructures, or other obstacles where distributed antenna systems arecommonly installed. Higher frequency wireless signals also providenarrow radiation patterns. Thus, remote units in distributed antennasystems may be arranged for line-of-sight (LOS) communications to allowfor higher frequencies for higher bandwidth. However, if remote unitsare provided in a LOS configuration and the remote units are alsoconfigured to support MIMO, multiple spatial streams received bymultiple receiver antennas in the remote units may be locked into arelative phase and/or amplitude pattern. This can lead to multiplereceived spatial streams periodically offsetting each other when thespatial streams are combined at MIMO receivers, leading to performancedegradation and reduced wireless coverage.

No admission is made that any reference cited herein constitutes priorart. Applicant expressly reserves the right to challenge the accuracyand pertinency of any cited documents.

SUMMARY

Components, systems, and methods for reducing location-dependentdestructive interference in distributed antenna systems (DASs) operatingin multiple-input, multiple-output (MIMO) configuration are disclosed.The DASs include remote units employing MIMO transmitters configured totransmit multiple data streams in MIMO configuration to MIMO receiversin wireless client devices. Destructive interference in a MIMO systemcan occur when two or more spatial streams transmitted from multipleMIMO antennas are locked into a relative phase and/or amplitude pattern,causing periodic destructive interferences when the two or more spatialstreams are combined at MIMO receivers in client devices. These issuescan occur due to lack of separation (i.e., phase, amplitude) in thereceived MIMO communications signals, especially with closely locatedMIMO transmitters configured for line-of-sight (LOS) communications.Thus, to provide spatial separation of MIMO communications signalsreceived by MIMO receivers in client devices, multiple MIMO transmittersin a remote unit in a DAS are each configured to employ multipletransmitter antennas, which are each configured to transmit in differentpolarization states. In certain embodiments, the amplitude of one of theMIMO communications signals is modified in one of the polarizationstates to further provide amplitude separation between the MIMOcommunications signals received by the MIMO receivers.

The components, systems, and methods for reducing location-dependentperiodic destructive interference in distributed antenna systemsoperating in MIMO configuration may significantly improve high-data ratewireless coverage without significant dependence on transmitter and/orreceive placement. This may allow for LOS communications to be moreeasily achieved between MIMO transmitters and MIMO receivers, especiallyfor higher frequency communications where LOS communications may berequired to reduce destructions to higher frequency signals by obstacleson the transmission path. High antenna isolation is not required in theMIMO receivers. No additional hardware component is required in the MIMOtransmitters or receivers as well. The improved MIMO performance andincreased coverage area can also allow higher frequency bands (e.g., 60GHz) to be used efficiently to provide multi-gigabit per second (Gbps)data access to client devices in indoor and outdoor environments.

One embodiment of the disclosure relates to a MIMO remote unitconfigured to wirelessly distribute MIMO communications signals towireless client devices in a distributed antenna system. The MIMO remoteunit comprises a first MIMO transmitter comprising a first MIMOtransmitter antenna configured to transmit MIMO communications signalsin a first polarization and a second MIMO transmitter antenna configuredto transmit MIMO communications signals in a second polarizationdifferent from the first polarization. The MIMO remote unit alsocomprises a second MIMO transmitter comprising a third MIMO transmitterantenna configured to transmit MIMO communications signals in the firstpolarization and a fourth MIMO transmitter antenna configured totransmit MIMO communications signals in the second polarization. Thefirst MIMO transmitter is configured to receive a first downlink MIMOcommunications signal at a first amplitude over a first downlinkcommunications medium, and transmit the first downlink MIMOcommunications signal wirelessly as a first electrical downlink MIMOcommunications signal over the first MIMO transmitter antenna in thefirst polarization. The first MIMO transmitter is also configured toreceive a second downlink MIMO communications signal at the firstamplitude over a second downlink communications medium, and transmit thesecond downlink MIMO communications signal wirelessly as a secondelectrical downlink MIMO communications signal over the second MIMOtransmitter antenna in the second polarization. The second MIMOtransmitter is configured to receive a third downlink MIMOcommunications signal at the first amplitude over a third downlinkcommunications medium, and transmit the third downlink MIMOcommunications signal wirelessly as a third electrical downlink MIMOcommunications signal over the third MIMO transmitter antenna in thefirst polarization. The second MIMO transmitter is also configured toreceive a fourth downlink MIMO communications signal over a fourthdownlink communications medium, and transmit the fourth downlink MIMOcommunications signal at a second amplitude modified from the firstamplitude, wirelessly as a fourth electrical downlink MIMOcommunications signal over the fourth MIMO transmitter antenna in thesecond polarization.

An additional embodiment of the disclosure relates to a method oftransmitting MIMO communications signals to wireless client devices in adistributed antenna system is provided. The method includes receiving afirst downlink MIMO communications signal at a first amplitude over afirst downlink communications medium. The method also includestransmitting the first downlink MIMO communications signal wirelessly asa first electrical downlink MIMO communications signal over a first MIMOtransmitter antenna in a first polarization. The method also includesreceiving a second downlink MIMO communications signal at the firstamplitude over a second downlink communications medium. The method alsoincludes transmitting the second downlink MIMO communications signalwirelessly as a second electrical downlink MIMO communications signalover a second MIMO transmitter antenna in a second polarization. Themethod also includes receiving a third downlink MIMO communicationssignal at the first amplitude over a third downlink communicationsmedium. The method also includes transmitting the third downlink MIMOcommunications signal wirelessly as a third electrical downlink MIMOcommunications signal over a third MIMO transmitter antenna in the firstpolarization. The method also includes receiving a fourth downlink MIMOcommunications signal over a fourth downlink communications medium. Themethod also includes transmitting the fourth downlink MIMOcommunications signal at a second amplitude modified from the firstamplitude, wirelessly as a fourth electrical downlink MIMOcommunications signal over a fourth MIMO transmitter antenna in thesecond polarization.

An additional embodiment of the disclosure relates to a distributedantenna system for distributing MIMO communications signals to wirelessclient devices. The distributed antenna system comprises a central unit.The central unit comprises a central unit transmitter configured toreceive a downlink communications signal. The central unit transmitteris also configured to transmit the received downlink communicationssignal as a first downlink MIMO communications signal over a firstdownlink communications medium, a second downlink MIMO communicationssignal over a second downlink communications medium, a third MIMOdownlink communications signal over a third downlink communicationsmedium, and a fourth downlink MIMO communications signal over a fourthdownlink communications medium.

This distributed antenna system also comprises a remote unit. The remoteunit comprises a first MIMO transmitter comprising a first MIMOtransmitter antenna configured to transmit MIMO communications signalsin a first polarization and a second MIMO transmitter antenna configuredto transmit MIMO communications signals in a second polarizationdifferent from the first polarization. The remote unit also comprises asecond MIMO transmitter comprising a third MIMO transmitter antennaconfigured to transmit MIMO communications signals in the firstpolarization and a fourth MIMO transmitter antenna configured totransmit MIMO communications signals in the second polarization. Thefirst MIMO transmitter is configured to receive a first downlink MIMOcommunications signal at a first amplitude over a first downlinkcommunications medium, and transmit the first downlink MIMOcommunications signal wirelessly as a first electrical downlink MIMOcommunications signal over the first MIMO transmitter antenna in thefirst polarization. The first MIMO transmitter is also configured toreceive a second downlink MIMO communications signal at the firstamplitude over a second downlink communications medium, and transmit thesecond downlink MIMO communications signal wirelessly as a secondelectrical downlink MIMO communications signal over the second MIMOtransmitter antenna in the second polarization. The second MIMOtransmitter is configured to receive a third downlink MIMOcommunications signal at the first amplitude over a third downlinkcommunications medium, and transmit the third downlink MIMOcommunications signal wirelessly as a third electrical downlink MIMOcommunications signal over the third MIMO transmitter antenna in thefirst polarization. The second MIMO transmitter is also configured toreceive a fourth downlink MIMO communications signal over a fourthdownlink communications medium, and transmit the fourth downlink MIMOcommunications signal at a second amplitude modified from the firstamplitude, wirelessly as a fourth electrical downlink MIMOcommunications signal over the fourth MIMO transmitter antenna in thesecond polarization. The remote unit also comprises at least oneamplitude adjustment circuit configured to amplitude adjust the fourthdownlink MIMO communications signal to the second amplitude.

The distributed antenna systems disclosed herein can be configured tosupport one or more radio-frequency (RF)-based services and/ordistribution of one or more digital data services. The remote units inthe distributed antenna systems may be configured to transmit andreceive wireless communications signals at one or more frequencies,including but not limited to extremely high frequencies (EHF) (i.e.,approximately 30 GHz-approximately 300 GHz). The distributed antennasystems may include, without limitation, wireless local area networks(WLANs). Further, as a non-limiting example, the distributed antennasystems may be an optical fiber-based distributed antenna system, butsuch is not required. An optical fiber-based distributed antenna systemmay employ Radio-over-Fiber (RoF) communications. The embodimentsdisclosed herein are also applicable to other remote antenna clustersand distributed antenna systems, including those that include otherforms of communications media for distribution of communicationssignals, including electrical conductors and wireless transmission. Forexample, the distributed antenna systems may include electrical and/orwireless communications mediums between a central unit and remote unitsin addition or in lieu of optical fiber communications medium. Theembodiments disclosed herein may also be applicable to remote antennaclusters and distributed antenna systems and may also include more thanone communications media for distribution of communications signals(e.g., digital data services, RF communications services). Thecommunications signals in the distributed antenna system may or may notbe frequency shifted.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and the claimshereof, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims. The accompanying drawings are included toprovide a further understanding and are incorporated in and constitute apart of this specification. The drawings illustrate one or moreembodiments, and together with the description serve to explainprinciples and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary distributed antenna system;

FIG. 2 is a schematic diagram of an exemplary multiple-input,multiple-output (MIMO) optical fiber-based distributed antenna system;

FIG. 3A is a top view diagram of a room having an exemplary MIMO antennasystem comprising two (2) MIMO transmitter antennas in line-of-sight(LOS) with two (2) MIMO receiver antennas to illustrate periodicdestructive interference in MIMO communications signals received in thesame frequency channel by the MIMO receiver antennas;

FIG. 3B is a graph illustrating exemplary measured periodic performancedegradations for a given placement distance between the MIMO transmitterantennas in the MIMO antenna system in FIG. 3A;

FIG. 3C is a graph illustrating an exemplary effective antenna coveragearea in proximity to the MIMO transmitter antennas in FIG. 3A;

FIG. 4 is a schematic diagram of an exemplary amplitude adjustmentcircuit for amplitude adjusting a downlink (DL) MIMO communicationssignal transmitted by a MIMO transmitter antenna in FIG. 2;

FIG. 5 is a flowchart illustrating an exemplary amplitude adjustmentprocess performed by the exemplary amplitude adjustment circuit in FIG.4 for amplitude adjusting a downlink (DL) MIMO communications signaltransmitted by a MIMO transmitter antenna in FIG. 2;

FIG. 6A is a schematic diagram of an exemplary MIMO optical fiber-baseddistributed antenna system employing a central unit employing a MIMOtransmitter configured to electrically amplitude adjust at least onetransmitted MIMO electrical downlink communications signal received andtransmitted by a remote unit employing multiple MIMO transmitters eachconfigured with multiple MIMO transmitter antennas configured totransmit in different polarization states;

FIG. 6B is a schematic diagram of an exemplary MIMO optical fiber-baseddistributed antenna system employing an amplitude adjustment circuit ofFIG. 4 in an optical downlink communications medium configured toprovide amplitude adjustment to at least one transmitted MIMO electricaldownlink communications signal received and transmitted by a remote unitemploying multiple MIMO transmitters each configured with multiple MIMOtransmitter antennas configured to transmit in different polarizationstates;

FIG. 6C is a schematic diagram of an exemplary MIMO optical fiber-baseddistributed antenna system employing remote units employing multipleMIMO transmitters each employing multiple MIMO transmitter antennasconfigured to transmit in different polarization states, wherein one ofthe MIMO electrical downlink communications signals transmitted by oneof the MIMO transmitters in a polarization state is electricallyamplitude adjusted;

FIG. 7 is a schematic diagram illustrating exemplary implementationoptions of an amplitude adjustment circuit in FIG. 4 in a central unitin FIGS. 6A-6C;

FIG. 8A is a graph illustrating exemplary MIMO communications signalwaveforms transmitted by a first MIMO transmitter antenna and a secondMIMO transmitter antenna of a MIMO transmitter in a remote unit in FIGS.6A-6C without amplitude adjustment;

FIG. 8B is a graph illustrating exemplary MIMO communications signalwaveforms transmitted by a first MIMO transmitter antenna and a secondMIMO transmitter antenna of a MIMO transmitter in a remote unit in FIGS.6A-6C with amplitude adjustment;

FIG. 8C is a graph illustrating exemplary measured periodic performancedegradation for a given placement distance between MIMO transmitterantennas in a MIMO transmitter in a remote unit in the distributedantenna system in FIGS. 6A-6C, when employing and not employingamplitude adjustment of at least one transmitted downlink communicationssignals;

FIG. 8D is a graph illustrating an exemplary effective antenna coverageversus placement distance between MIMO transmitter antennas in a MIMOtransmitter in a remote unit in the distributed antenna system in FIGS.6A-6C, for a given placement distance between MIMO receiver antennas,when employing and not employing amplitude adjustment of at least onetransmitted downlink communications signal; and

FIG. 9 is a schematic diagram of a generalized representation of anexemplary controller that can be included in any central unit, remoteunits, wireless client devices, and/or any other components ofdistributed antenna systems to reduce or eliminate issues of periodicdestructive interference in transmitted MIMO electrical downlinkcommunications signals, wherein the exemplary computer system is adaptedto execute instructions from an exemplary computer readable medium.

DETAILED DESCRIPTION

Components, systems, and methods for reducing location-dependentdestructive interference in distributed antenna systems (DASs) operatingin multiple-input, multiple-output (MIMO) configuration are disclosed.The DASs include remote units employing MIMO transmitters configured totransmit multiple data streams in MIMO configuration to MIMO receiversin wireless client devices. Destructive interference in a MIMO systemcan occur when two or more spatial streams transmitted from multipleMIMO antennas are locked into a relative phase and/or amplitude pattern,causing periodic destructive interferences when the two or more spatialstreams are combined at MIMO receivers in client devices. These issuescan occur due to lack of separation (i.e., phase, amplitude) in thereceived MIMO communications signals, especially with closely locatedMIMO transmitters configured for line-of-sight (LOS) communications.Thus, to provide spatial separation of MIMO communications signalsreceived by MIMO receivers in client devices, multiple MIMO transmittersin a remote unit in a DAS are each configured to employ multipletransmitter antennas, which are each configured to transmit in differentpolarization states. In certain embodiments, the amplitude of one of theMIMO communications signals is modified in one of the polarizationstates to further provide amplitude separation between the MIMOcommunications signals received by the MIMO receivers. Variousembodiments will be explained by the following examples.

Before discussing examples of components, systems, and methods forreducing location-dependent destructive interference in distributedantenna systems operating in MIMO configuration starting at FIG. 4, anexemplary distributed antenna system is described in regard to FIGS.1-3C. In this regard, FIG. 1 is a schematic diagram of a conventionaldistributed antenna system 10. The distributed antenna system 10 is anoptical fiber-based distributed antenna system. The distributed antennasystem 10 is configured to create one or more antenna coverage areas forestablishing communications with wireless client devices located in theradio frequency (RF) range of the antenna coverage areas. In anexemplary embodiment, the distributed antenna system 10 may provide RFcommunication services (e.g., cellular services). As illustrated, thedistributed antenna system 10 includes a central unit 12, one or moreremote units 14, and an optical fiber 16 that optically couples thecentral unit 12 to the remote unit 14. The central unit 12 may also bereferred to as a head-end unit. The remote unit 14 is a type of remotecommunications unit, and may also be referred to as a “remote antennaunit.” In general, a remote communications unit can support wirelesscommunications or wired communications, or both. The central unit 12 isconfigured to receive communications over downlink electrical RF signals18D from a source or sources, such as a network or carrier as examples,and provide such communications to the remote unit 14. The central unit12 is also configured to return communications received from the remoteunit 14, via uplink electrical RF signals 18U, back to the source orsources. In this regard, in this embodiment, the optical fiber 16includes at least one downlink optical fiber 16D to carry signalscommunicated from the central unit 12 to the remote unit 14 and at leastone uplink optical fiber 16U to carry signals communicated from theremote unit 14 back to the central unit 12.

One downlink optical fiber 16D and one uplink optical fiber 16U could beprovided to support multiple full-duplex channels each usingwave-division multiplexing (WDM), as discussed in U.S. patentapplication Ser. No. 12/892,424, entitled “Providing Digital DataServices in Optical Fiber-based Distributed Radio Frequency (RF)Communications Systems, And Related Components and Methods,”incorporated herein by reference in its entirety. Other options for WDMand frequency-division multiplexing (FDM) are also disclosed in U.S.patent application Ser. No. 12/892,424, any of which can be employed inany of the embodiments disclosed herein. Further, U.S. patentapplication Ser. No. 12/892,424 also discloses distributed digital datacommunications signals in a distributed antenna system which may also bedistributed in the distributed antenna system 10 either in conjunctionwith the RF communications signals or not.

The distributed antenna system 10 has an antenna coverage area 20 thatcan be disposed around the remote unit 14. The antenna coverage area 20of the remote unit 14 forms an RF coverage area 21. The central unit 12is adapted to perform or to facilitate any one of a number ofRadio-over-Fiber (RoF) applications, such as RF identification (RFID),wireless local-area network (WLAN) communication, or cellular phoneservice. Shown within the antenna coverage area 20 is a client device 24in the form of a mobile device, which may be a cellular telephone as anexample. The client device 24 can be any device that is capable ofreceiving RF communications signals. The client device 24 includes anantenna 26 (e.g., a wireless card) adapted to receive and/or sendelectromagnetic RF signals.

With continuing reference to FIG. 1, to communicate the electrical RFsignals over the downlink optical fiber 16D to the remote unit 14, to inturn be communicated to the client device 24 in the antenna coveragearea 20 formed by the remote unit 14, the central unit 12 includes aradio interface in the form of an electrical-to-optical (E/O) converter28. The E/O converter 28 converts the downlink electrical RF signals 18Dto downlink optical RF signals 22D to be communicated over the downlinkoptical fiber 16D. The remote unit 14 includes an optical-to-electrical(0/E) converter 30 to convert the received downlink optical RF signals22D back to electrical RF signals to be communicated wirelessly throughan antenna 32 of the remote unit 14 to the client device 24 located inthe antenna coverage area 20.

Similarly, the antenna 32 is also configured to receive wireless RFcommunications from the client device 24 in the antenna coverage area20. In this regard, the antenna 32 receives wireless RF communicationsfrom the client device 24 and communicates electrical RF signalsrepresenting the wireless RF communications to an E/O converter 34 inthe remote unit 14. The E/O converter 34 converts the electrical RFsignals into uplink optical RF signals 22U to be communicated over theuplink optical fiber 16U. An 0/E converter 36 provided in the centralunit 12 converts the uplink optical RF signals 22U into uplinkelectrical RF signals, which can then be communicated as uplinkelectrical RF signals 18U back to a network or other source.

As noted, one or more of the network or other sources can be a cellularsystem, which may include a base station or base transceiver station(BTS). The BTS may be provided by a second party such as a cellularservice provider, and can be co-located or located remotely from thecentral unit 12.

In a typical cellular system, for example, a plurality of BTSs isdeployed at a plurality of remote locations to provide wirelesstelephone coverage. Each BTS serves a corresponding cell and when amobile client device enters the cell, the BTS communicates with themobile client device. Each BTS can include at least one radiotransceiver for enabling communication with one or more subscriber unitsoperating within the associated cell. As another example, wirelessrepeaters or bi-directional amplifiers could also be used to serve acorresponding cell in lieu of a BTS. Alternatively, radio input could beprovided by a repeater, picocell, or femtocell, as other examples. In aparticular exemplary embodiment, cellular signal distribution in thefrequency range from 400 MHz to 2.7 GHz is supported by the distributedantenna system 10.

Although the distributed antenna system 10 in FIG. 1 allows fordistribution of radio frequency (RF) communications signals; thedistributed antenna system 10 is not limited to distribution of RFcommunications signals. Data communications signals, including digitaldata signals, for distributing data services could also be distributedin the distributed antenna system 10 in lieu of or in addition to RFcommunications signals. Also note that while the distributed antennasystem 10 in FIG. 1 discussed below includes distribution ofcommunications signals over optical fiber, the distributed antennasystem 10 is not limited to distribution of communications signals overoptical fiber. Distribution media could also include, but are notlimited to, coaxial cable, twisted-pair conductors, wirelesstransmission and reception, and any combination thereof. Also, anycombination can be employed that also involves optical fiber forportions of the distributed system.

A distributed antenna system, including the distributed antenna system10 in FIG. 1, can be configured in MIMO configuration for MIMOoperation. In this regard, FIG. 2 illustrates a schematic diagram of anexemplary MIMO optical fiber-based distributed antenna system 40(hereinafter referred to as “MIMO distributed antenna system 40”). TheMIMO distributed antenna system 40 is configured to operate in MIMOconfiguration. MIMO technology involves the use of multiple antennas atboth a transmitter and receiver to improve communication performance. Inthis regard, a central unit 42 is provided that is configured todistribute downlink communications signals to one or more remote units44. FIG. 2 only illustrates one remote unit 44, but note that aplurality of remote units 44 is typically provided. The remote units 44are configured to wirelessly communicate the downlink communicationsignals to one or more client devices 46 that are in communication rangeof the remote unit 44. The remote units 44 may also be referred to as“remote antenna units 44” because of their wireless transmission overantenna functionality. The remote unit 44 is also configured to receiveuplink communication signals from the client devices 46 to bedistributed to the central unit 42. In this embodiment, an optical fibercommunications medium 47 comprising at least one downlink optical fiber48D and at least one uplink optical fiber 48U is provided tocommutatively couple the central unit 42 to the remote units 44. Thecentral unit 42 is also configured to receive uplink communicationsignals from the remote units 44 via the optical fiber communicationsmedium 47, although more specifically over the at least one uplinkoptical fiber 48U. The client device 46 in communication with the remoteunit 44 can provide uplink communication signals to the remote unit 44which are then distributed over the optical fiber communications medium47 to the remote unit 44 to be provided to a network or other source,such as a base station for example.

With continuing reference to FIG. 2, more detail will be discussedregarding the components of the central unit 42, the remote unit 44, andthe client device 46 and the distribution of downlink communicationssignals. The central unit 42 is configured to receive electricaldownlink MIMO communication signals 50D from outside the MIMOdistributed antenna system 40 in a signal processor 52 and provideelectrical uplink communications signals 50U received from clientdevices 46, to other systems. The signal processor 52 is configured toprovide the electrical downlink communication signals 50D to a mixer 60,which may be an IQ signal mixer in this example. The mixer 60 in thisembodiment is configured to convert the electrical downlink MIMOcommunication signals 50D to IQ signals. The mixer 60 is driven by afrequency signal 56 that is provided by a local oscillator 58. Frequencyconversion is optional. In this embodiment, it is desired to up-convertthe frequency of the electrical downlink MIMO communication signals 50Dto a higher frequency to provide electrical downlink MIMO communicationsignals 66D to provide for a greater bandwidth capability beforedistributing the electrical downlink MIMO communications signals 66D tothe remote units 44. For example, the up-conversion carrier frequencymay be provided as an extremely high frequency (e.g. approximately 30GHz to 300 GHz).

With continuing reference to FIG. 2, because the communication mediumbetween the central unit 42 and the remote unit 44 is the optical fibercommunications medium 47, the electrical downlink MIMO communicationsignals 66D are converted to optical signals by an electro-opticalconverter 67. The electro-optical converter 67 includes components toreceive a light wave 68 from a light source 70, such as a laser. Thelight wave 68 is modulated by the frequency oscillations in theelectrical downlink MIMO communication signals 66D to provide opticaldownlink MIMO communication signals 72D to be communicated over thedownlink optical fiber 48D to the remote unit 44. The electro-opticalconverter 67 may be provided so that the electrical downlink MIMOcommunication signals 66D are provided as radio-over-fiber (RoF)communications signals over the downlink optical fiber 48D.

With continuing reference to FIG. 2, the optical downlink MIMOcommunication signals 72D are received by an optical bi-directionalamplifier 74, which is then provided to a MIMO splitter 76 in the remoteunit 44. The MIMO splitter 76 is provided so that the optical downlinkMIMO communication signals 72D can be split among two separatecommunication paths 77(1), 77(2) to be radiated over two separate MIMOtransmitter antennas 78(1), 78(2) provided in two separate MIMOtransmitters 79(1), 79(2) configured in MIMO configuration. The MIMOsplitter 76 in the remote unit 44 is an optical splitter since thereceived optical downlink MIMO communication signals 72D are opticalsignals. In each communication path 77(1), 77(2), optical-to-electricalconverters 80(1), 80(2) are provided to convert the optical downlinkMIMO communication signals 72D to electrical downlink MIMO communicationsignals 82D(1), 82D(2). In this embodiment, as will be discussed in moredetail below, an amplitude adjustment circuit 84 is provided in one ofthe transmission paths 77(1), 77(2) to provide amplitude adjustment inone of the optical downlink MIMO communication signals 72D(1), 72D(2)transmitted over one of the MIMO transmitter antennas 78(1), 78(2) tohelp reduce or eliminate periodic destructive interferences whenreceived electrical downlink MIMO communication signals 82D are combinedat the client device 46.

A destructive interference occurs when the electrical downlink MIMOcommunication signals 82D(1), 82D(2) are locked into a relative phaseand/or amplitude pattern, causing them to cancel each other whencombined at MIMO receivers 85(1), 85(2). Because the electrical downlinkMIMO communication signals 82D(1), 82D(2) are periodic radio frequencywaves, the destructive interference also becomes periodic as result.When physical obstacles (e.g., buildings, walls, trees, vehicles, etc.)standing in radio transmission paths between the MIMO transmitterantennas 78(1), 78(2) and the MIMO receiver antennas 86(1), 86(2), theelectrical downlink MIMO communication signals 82D(1), 82D(2)transmitted by the MIMO transmitter antennas 78(1), 78(2) typicallyarrive at the MIMO receiver antennas 86(1), 86(2) from differentdirections and/or angles (also known as “multipath”) due to reflectionsfrom the physical obstacles. Due to multipath effect, the electricaldownlink MIMO communication signals 82D(1), 82D(2) transmitted by theMIMO transmitter antennas 78(1), 78(2) may arrive at the MIMO receiverantennas 86(1), 86(2) with slight delays among each other, resulting innatural phase shifts between the electrical downlink MIMO communicationsignals 82D(1), 82D(2). Further, the amplitudes of the electricaldownlink MIMO communication signals 82D(1), 82D(2) may also be modifieddue to different reflection angles caused by different obstacles alongdifferent transmission paths. In this regard, multipath acts to break upthe locked-in phase and/or amplitude pattern among the electricaldownlink MIMO communication signals 82D(1), 82D(2) transmitted by theMIMO transmitter antennas 78(1), 78(2) and, thus, helps mitigateperiodic destructive interferences at MIMO receivers 85(1), 85(2).However, when a millimeter wave radio frequency band (e.g., 60 GHz) isemployed as the carrier frequency between the MIMO transmitter antennas78(1), 78(2) and the MIMO receiver antennas 86(1), 86(2), there cannotbe any physical obstacle stand in the radio transmission path. This isbecause higher frequency signals like a 60 GHz signal are inherentlyincapable of penetrating or bouncing off physical obstacles. To preventmillimeter wave radio frequency signals from being blocked by physicalobstacles, the MIMO transmitter antennas 78(1), 78(2) and the MIMOreceiver antennas 86(1), 86(2) must be configured in a line-of-sight(LOS) arrangement, which is further elaborated in FIGS. 3A-3C. With theLOS arrangement, multipath becomes non-existent between the MIMOtransmitter antennas 78(1), 78(2) and the MIMO receiver antennas 86(1),86(2). Therefore, periodic destructive interferences often occur whenthe electrical downlink MIMO communication signals 82D(1), 82D(2) arecombined at the MIMO receivers 85(1), 85(2).

With continuing reference to FIG. 2, the client device 46 includes twoMIMO receivers 85(1), 85(2) that include MIMO receiver antennas 86(1),86(2) also configured in MIMO configuration. The MIMO receiver antennas86(1), 86(2) are configured to receive the electrical downlink MIMOcommunication signals 82D(1), 82D(2) wirelessly from the remote unit 44.Mixers 88(1), 88(2) are provided and coupled to the MIMO receiverantennas 86(1), 86(2) in the client device 46 to provide frequencyconversion of the electrical downlink MIMO communication signals 82D(1),82D(2). In this regard, a local oscillator 90 is provided that isconfigured to provide oscillation signals 92(1), 92(2) to the mixers88(1), 88(2), respectively, for frequency conversion. In thisembodiment, the electrical downlink MIMO communications signals 82D(1),82D(2) are down converted back to their native frequency as received bythe central unit 42. The down converted electrical downlink MIMOcommunication signals 82D(1), 82D(2) are then provided to a signalanalyzer 94 in the client device 46 for any processing desired.

FIG. 3A illustrates a top view of a room 100 employing the exemplaryMIMO distributed antenna system 40 in FIG. 2 to discuss performance ofMIMO communications as affected by antenna placement. As illustrated inFIG. 3A, the two MIMO transmitter antennas 78(1), 78(2) of the remoteunit 44 are shown as being located in the room 100. Similarly, a clientdevice 46 is shown with its two MIMO receiver antennas 86(1), 86(2)configured to receive the electrical downlink MIMO communication signals82D(1), 82D(2) from the two MIMO transmitters 81(1), 81(2) (shown inFIG. 2) in MIMO configuration. The two MIMO transmitter antennas 78(1),78(2) and two MIMO receiver antennas 86(1), 86(2) are placed accordingto the LOS arrangement. The LOS arrangement ensures that the electricaldownlink MIMO communication signals 82D(1), 82D(2) from the two MIMOtransmitters 81(1), 81(2) are directed towards the two MIMO receiverantennas 86(1), 86(2), even if the electrical downlink MIMOcommunication signals 82D(1), 82D(2) are reflected on the downlinkpropagation path. In other words, the LOS arrangement does not stop thetwo MIMO receiver antennas 86(1), 86(2) from receiving reflectedsignals. The MIMO transmitter antennas 78(1), 78(2) in the MIMOtransmitters 81(1), 81(2) in the remote unit 44 are separated by adistance D₁. The MIMO receiver antennas 86(1), 86(2) in the clientdevice 46 are separated by a distance D₂. In absence of multipath due tothe LOS arrangement, issues can arise, due to destructive interference,with MIMO algorithm being able to solve the channel matrix for receivedelectrical downlink MIMO communication signals 82D(1), 82D(2) at theclient device 46 as a function of the distance D₁ between the MIMOtransmitter antennas 78(1), 78(2) in the remote unit 44, the distance D₂between MIMO receiver antennas 86(1), 86(2) in the client device 46, andthe distance D₃ between remote unit 44 and the client device 46. Theseissues are also referred to herein as location-dependent destructiveinterference issues.

Location-dependent destructive interference for the received electricaldownlink MIMO communication signals 82D(1), 82D(2) can negatively affectMIMO communications performance. These issues with electrical downlinkMIMO communication signals 82D(1), 82D(2) received by the MIMO receiverantennas 86(1), 86(2) can occur due to lack of separation (e.g., phase,amplitude) in the received electrical downlink MIMO communicationsignals 82D(1), 82D(2), especially in LOS communications. To illustratethe effect of these issues, FIG. 3B illustrates a graph 102 illustratingthe exemplary measured performance degradation for a given placementdistance between the MIMO transmitter antennas 78(1), 78(2) in FIG. 3A.The graph 102 in FIG. 3B illustrates the capacity on the y-axis inGigabits per second (Gbps) versus the MIMO transmitter antennas 78(1),78(2) separation distance D₁ in centimeters. As illustrated in the graph102, at separation distances D₁ of approximately 42 centimeters (cm) and85 cm, the communications capacity illustrated by a capacity curve 104is periodically degraded due to periodic destructive interferencesbetween the received electrical downlink MIMO communication signals82D(1), 82D(2). Similarly, a MIMO condition number curve 106 in FIG. 3Balso illustrates the effect periodic destructive interferences betweenthe received electrical downlink MIMO communication signals 82D(1),82D(2), which is complementary to the capacity curve 104.

FIG. 3C illustrates a graph 108 representing an exemplary effectivecommunication coverage area provided by the MIMO distributed antennasystem 40 in FIG. 2 according to the MIMO transmitter antennas 78(1),78(2), separation distance D₁, the MIMO receiver antennas 86(1), 86(2),separation distance D₂ in FIG. 3A, and distance D₃ therebetween. Asillustrated in FIG. 3C, a desired antenna coverage area 109 is shown asbeing provided by the area formed inside a boundary line 110. However,an actual communication coverage area 113 for the remote unit 44 isprovided inside the boundary line 112, illustrating the effect inreduction communication range of the remote unit 44.

To address these issues, FIGS. 4-8D are provided to illustrate exemplarydistributed antenna systems configured to reduce location-dependentdestructive interference in distributed antenna systems operating inMIMO configuration. In these embodiments, to provide spatial separationof MIMO communication signals received by MIMO receivers in clientdevices, multiple MIMO transmitters in a remote unit are each configuredto employ multiple transmitter antennas. The multiple transmitterantennas are each configured to transmit communications signals indifferent polarization states. In certain embodiments, one of the MIMOcommunications signals is amplitude adjusted in one of the polarizationstates to provide amplitude separation between MIMO communicationsignals received by the MIMO receivers.

In this regard, FIG. 4 illustrates an exemplary amplitude adjustmentcircuit for amplitude adjusting a DL MIMO communication signaltransmitted by a MIMO transmitter antenna 78 in FIG. 2. The exemplaryamplitude adjustment circuit 120 comprises a signal controller 122 andan amplitude adjustment logic 124. As a non-limiting example, theamplitude adjustment logic 124 may be implemented by a hardwarecomponent, a software function, or a combination of both. In anothernon-limiting example, the signal controller 122 may be a digitalbaseband processor, a digital signal processor, a MIMO controller, or ageneral-purpose processor (e.g., central processing unit (CPU)). Thesignal controller 122 receives a MIMO performance measurement 126 on anuplink reception path (not shown). The signal controller 122 isconfigured to compare the MIMO performance measurement 126 with apre-determined MIMO performance threshold. If the MIMO performancemeasurement 126 indicates a MIMO performance level is below thepre-determined MIMO performance threshold, the signal controller 122 isfurther configured to provide an amplitude adjustment signal 128 to theamplitude adjustment logic 124 to perform amplitude adjustment on adownlink MIMO communication signal 130. The amplitude adjustment logic124 in turn performs amplitude adjustment on the downlink communicationsignal 130 received from a downlink transmission path (not shown). Theamplitude adjustment circuit thus produces an amplitude-adjusteddownlink communication signal 134 that is sent to a MIMO transmitterantenna on the downlink transmission path (not shown). In a non-limitingexample, if the DL MIMO communication signal 130 has an originalamplitude x, the amplitude adjustment logic 124 may produce a modifiedamplitude y that is different from the original amplitude x for theamplitude-adjusted downlink communication signal 134.

With continuing reference to FIG. 4, FIG. 5 illustrates an exemplaryamplitude adjustment process performed by the exemplary amplitudeadjustment circuit in FIG. 4. FIGS. 2 and 4 are referenced in connectionwith FIG. 5 and will not be re-described herein. The amplitudeadjustment process 140 is invoked when wireless communication starts(block 142). The signal controller 122 receives and processes a MIMOperformance measurement (block 144) and compares the MIMO performancemeasurement with a pre-determined threshold (block 146). If the MIMOperformance measurement is above the pre-determined threshold, itindicates that the MIMO transmitter antennas 78 are performing asexpected. In this case, the signal controller 122 will not take anyaction and awaits a next MIMO performance measurement. If, however, theMIMO performance measurement is below the pre-determined threshold, itis an indication that the MIMO transmitter antennas 78 are notperforming as expected. Under such circumstance, the signal controller122 will instruct the amplitude adjustment logic 124 to modify theamplitude of the downlink MIMO communication signal 130 (block 148). Theamplitude adjustment process 140 repeats the step of comparing MIMOperformance measurement against the pre-determined threshold (block 146)and the step of amplitude adjustment (block 148) until the next MIMOperformance measurement is above the pre-determined threshold.

In this regard, FIGS. 6A-6C illustrate alternative MIMO distributedantenna systems 40(1)-40(3) similar to the MIMO distributed antennasystem 40 in FIG. 2. FIGS. 6A-6C respectively illustrate three (3)different downlink signal processing stages in the MIMO distributedantenna systems 40(1)-40(3) wherein the amplitude adjustment circuit 120may be provided. The MIMO distributed antenna systems 40(1)-40(3) inFIGS. 6A-6C are configured to reduce or eliminate periodic destructiveinterferences between received downlink communication signals at a MIMOreceiver in a client device so as to reduce or eliminate performancedegradation such as shown in FIGS. 3B and 3C above. The MIMO distributedantenna systems 40(1)-40(3) may include the same components in the MIMOdistributed antenna system 40 in FIG. 2 unless otherwise noted in FIGS.6A-6C. Elements of FIG. 4 are referenced in connection with FIGS. 6A-6Cand will not be re-described herein.

With reference to FIG. 6A, a central unit 42(1) is configured to receivethe electrical downlink MIMO communications signals 50D as discussed inregard to FIG. 2. However, a signal processor 52(1) is configured tosplit the electrical downlink MIMO communications signals 50D into four(4) electrical downlink MIMO communications signals 50D(1)-50D(4) overfour separate channels. As a first option, an amplitude adjustmentcircuit 120(1) is provided in the central unit 42(1) to amplitude adjustat least one of the electrical downlink MIMO communications signals 50D.Note that although the electrical downlink MIMO communications signal50D(4) is amplitude adjusted in this example, any other(s) downlink MIMOcommunications signal(s) 50D(1)-50D(3) could be amplitude adjusted aswell. The amplitude adjustment circuit 120(1) may be programmed orcontrolled by the signal controller 122 to provide a pre-determinedlevel of amplitude adjustment, if desired. Turning back to the centralunit 42(1), electro-optical converters 67(1)-67(4) are provided toconvert the electrical downlink MIMO communications signals50D(1)-50D(4) into optical downlink MIMO communications signals72D(1)-72D(4) provided over optical fiber communications medium 47(1).

With continuing reference to FIG. 6A, the remote unit 44(1) includes twoMIMO transmitters 154(1), 154(2) in MIMO configuration. However, theMIMO transmitters 154(1), 154(2) each include two MIMO transmitterantennas 156(1)(1), 156(1)(2), and 156(2)(1), 156(2)(2). The first MIMOtransmitter 154(1) includes the first MIMO transmitter antenna 156(1)(1)configured to radiate the first electrical downlink MIMO communicationssignals 82D(1) (after conversion from optical to electrical signals) ina first polarization 158(1). The first MIMO transmitter 154(1) alsoincludes the second MIMO transmitter antenna 156(1)(2) configured toradiate the second electrical downlink MIMO communications signal 82D(2)in a second polarization 158(2) different from the first polarization158(1). In this manner, the first and second electrical downlink MIMOcommunications signals 82D(1), 82D(2) can be received by two differentMIMO receiver antennas 160(1), 160(2) in MIMO receivers 162(1), 162(2),respectively, each configured to receive signals in differentpolarizations 158(1), 158(2) among the first and second polarizations158(1), 158(2) without experiencing periodic destructive interferences.Thus, the MIMO receivers 162(1), 162(2) can receive the first and secondelectrical downlink MIMO communications signal 82D(1), 82D(2) indifferent polarizations 158(1), 158(2), respectively, from the firstMIMO transmitter 154(1) so that a MIMO algorithm can solve the channelmatrix for the first and second electrical downlink MIMO communicationssignal 82D(1), 82D(2). In this embodiment, the first polarization 158(1)is configured to be orthogonal to the second polarization 158(2) tomaximize spectral efficiency and minimize cross talk between theelectrical downlink MIMO communications signals 82D(1), 82D(2) at theMIMO receivers 162(1), 162(2), but this configuration is not required.

With continuing reference to FIG. 6A, the second MIMO transmitter 154(2)in the remote unit 44(1) includes a third MIMO transmitter antenna156(2)(1) configured to radiate the third electrical downlink MIMOcommunications signals 82D(3) (after conversion from optical toelectrical signals) in the first polarization 158(1). The second MIMOtransmitter 154(2) also includes the fourth MIMO transmitter antenna156(2)(2) configured to radiate the fourth electrical downlink MIMOcommunications signal 82D(4) in the second polarization 158(2) differentfrom the first polarization 158(1). In this manner, the third and fourthelectrical downlink MIMO communications signals 82D(3), 82D(4) can alsobe received by the two different MIMO receiver antennas 160(1), 160(2)in MIMO receivers 162(1), 162(2), respectively, each configured toreceive signals in different polarizations 158(1), 158(2) among thefirst and second polarizations 158(1), 158(2). Thus, the MIMO receivers162(1), 162(2) can receive the third and fourth electrical downlink MIMOcommunications signal 82D(3), 82D(4) in different polarizations,respectively, from the second MIMO transmitter 154(2) between the thirdand fourth electrical downlink MIMO communications signal 82D(3),82D(4). The electrical downlink MIMO communications signals82D(1)-82D(4) are received by the MIMO receivers 162(1), 162(2) andprovided to a signal processor 164 and a MIMO processor 166 forprocessing.

As previously discussed above, the amplitude adjustment circuit 120(1)is provided in the central unit 42(1) to amplitude adjust the electricaldownlink MIMO communications signal 50D(4) The amplitude adjustment inthe above example in turn causes the second and fourth electricaldownlink MIMO communications signals 82D(2), 82D(4) to be received bythe second MIMO receiver antennas 160(2) to have a small but sufficientamplitude difference. Further, the second and fourth electrical downlinkMIMO communications signals 82D(2), 82D(4) are also received by thesecond MIMO receiver antenna 160(2) in the second polarization 158(2),which is different from the first and third electrical downlink MIMOcommunications signals 82D(1), 82D(3) received by the first MIMOreceiver 162(1) in the first polarization 158(1). This combination ofamplitude adjustment and MIMO transmitter antenna polarization canreduce or eliminate periodic destructive interferences between the firstand the third electrical downlink MIMO communications signals 82D(1),82D(3) being received by the first MIMO receiver 162(1) and between thesecond and the fourth electrical downlink MIMO communications signals82D(2), 82D(4) being received by the second MIMO receiver 162(2).

As previously stated above, the amplitude adjustment circuit 120 can beprovided in other downlink signal processing stages of the MIMOdistributed antenna system 40 other than in the central unit, asprovided in the MIMO distributed antenna system 40(1) in FIG. 6A. Inthis regard, FIG. 6B is a schematic diagram of another MIMO opticalfiber-based distributed antenna system 40(2) (“MIMO distributed antennasystem 40(2)”) employing an amplitude adjustment circuit 120(2) in theoptical fiber communications medium 47(1). The amplitude adjustmentcircuit 120(2) can be tunable to allow for the amplitude adjustment tobe controlled and tuned. The amplitude adjustment circuit 120(2) may bean optical attenuator or amplifier that makes the amplitude of theoptical downlink MIMO communications signal 72D(1) smaller or larger,respectively, than the other downlink optical fibers of the opticalfiber communications medium 47(1). Common elements between the MIMOdistributed antenna system 40(1) in FIG. 6A and the MIMO distributedantenna system 40(2) in FIG. 6B are noted with common element numbersand will not be re-described. In this embodiment, the amplitudeadjustment circuit 120(2) is configured to optically amplitude adjustthe optical downlink MIMO communications signal 72D(4) received andtransmitted by the second MIMO transmitter 154(2) to the client device46(1). The central unit 42(2) in FIG. 6B does not include the amplitudeadjustment circuit 120(1) to amplitude shift downlink electricalcommunications signals like provided in the central unit 42(1) in FIG.6A.

As previously discussed above with regard to FIGS. 6A and 6B, theamplitude adjustment circuit 120 can be provided in the central unit42(1) and/or the optical fiber communications medium 47(1) to amplitudeadjust the electrical downlink MIMO communications signal 50D(4). Inthis regard, FIG. 6C is a schematic diagram of another MIMO opticalfiber-based distributed antenna system 40(3) (“MIMO distributed antennasystem 40(3)”) employing an amplitude adjustment circuit 120(3) in theform of an antenna power attenuator or amplifier in the remote unit44(2). Common elements between the MIMO distributed antenna system 40(3)in FIG. 6C and the MIMO distributed antenna systems 40(1), 40(2) inFIGS. 6A and 6B are noted with common element numbers and will not bere-described. In this embodiment, a signal processor 174 in the remoteunit 44(2) receives the optical downlink MIMO communications signals72D(1)-72D(4) and converts these signals into electrical downlink MIMOcommunications signals 82D(1)-82D(4) in an optical-to-electricalconverter. The amplitude adjustment circuit 120(3) is configured toelectrically amplitude adjust the electrical downlink MIMOcommunications signal 82D(4) received and transmitted by the second MIMOtransmitter 154(2) in the remote unit 44(2) to the client device 46(1)so that periodic destructive interferences resulting from LOSarrangement can be reduced or eliminated at the MIMO receivers 162(1),162(2).

With reference back to FIG. 4, the amplitude adjustment circuit 120includes the amplitude adjustment logic 124 configured to make amplitudeadjustment on the DL MIMO communication signal 130 based on theamplitude adjustment signal 128 received from the signal controller 122.Also with reference back to FIG. 6A, the amplitude adjustment circuit120(1) is provided in the central unit 42(1) of the MIMO distributedantenna system 40(1) to electronically amplitude adjust at least one ofthe electrical downlink MIMO communications signals 50D(4). Theamplitude adjustment circuit 120(1) may be configured to provideamplitude adjustment in a plurality of ways depending on how theamplitude adjustment logic 124 is implemented. In this regard, FIG. 7illustrates exemplary implementation options of the amplitude adjustmentcircuit 120(1) in the central unit 42(1). Elements of FIGS. 4 and 6A arereferenced in connection with FIG. 7 and will not be re-describedherein. Common elements between the central unit 42(1) in FIG. 6A andthe central unit 42(3) in FIG. 7 are noted with common element numbersand will not be re-described.

With reference to FIG. 7, an amplitude adjustment circuit 120(4) in thecentral unit 42(3) may be implemented in three different options120(4)(1), 120(4)(2), and 120(4)(3). An amplitude adjustment option120(4)(1) comprises a signal controller 122(1) configured to receive acontrol signal 170(1) from a baseband signal processing module (notshown) and provide an amplitude adjustment signal 128(1) to an amplitudeadjustment logic 124(1). In response to receiving the amplitudeadjustment signal 128(1), the amplitude adjustment logic 124(1) performsamplitude adjustment on a downlink MIMO communications signal 50D(4)received from a signal processor 52(1). The amplitude adjustment logic124(1), in this non-limiting example, is a tunable attenuator or avariable gain amplifier (VGA) that may be electronically controlled bythe signal controller 122(1) to reduce or increase amplitude of thedownlink MIMO communications signal 50D(4). An amplitude adjusteddownlink MIMO communications signal 172 is received by anelectrical/optical converter 67(4) and converted into an opticaldownlink MIMO communications signal 72D(4) (not shown) for transmissionover the fiber communication medium 47(1) (not shown). Note thatalthough the electrical downlink MIMO communications signal 50D(4) isamplitude adjusted in this example, any other(s) downlink MIMOcommunications signal(s) 50D(1)-50D(3) could be amplitude adjusted inthe same way as the downlink MIMO communications signal 50D(4).Alternatively, an amplitude adjustment option 120(4)(2) comprises anamplitude adjustment logic 124(2) configured to provide amplitudeadjustment on the downlink MIMO communications signal 50D(4) receivedfrom the signal processor 52(1) by adjusting bias signal of a laserdiode. Common elements between the amplitude adjustment option 120(4)(1)and the amplitude adjustment option 120(4)(2) are noted with commonelement numbers and will not be re-described.

With continuing reference to FIG. 7, a third amplitude adjustment option120(4)(3) comprises an amplitude adjustment logic 124(3) that is anoptical modulator. In a non-limiting example, the amplitude adjustmentlogic 124(3) may be a Mach-Zehnder modulator (MZM) or anelectro-absorption modulator (EAM). A bias voltage signal 175 isprovided to the amplitude adjustment logic 124(3) from a laser diode176. Biasing in electronic circuits is a method of establishing variouspre-determined voltage or current pointes to provide proper operatingconditions in the amplitude adjustment logic 124(3). In a typical EAM,for example, a 0.3 volt (V) variation in bias signal results inapproximately three (3) decibel (dB) amplitude variation in an outputsignal. Thus, by providing the bias voltage signal 175 to the amplitudeadjustment logic 124(3), the amplitude of the downlink MIMOcommunications signal 50D(4) received from the signal processor 52(1)may be adjusted. Other common elements among the amplitude adjustmentcircuit 120(4)(1), 120(4)(2), 120(4)(3) are noted with common elementnumbers and will not be re-described.

To help visualize the concept of amplitude adjustment, FIGS. 8A-8B areprovided. Elements of FIGS. 6A-6C are referenced in connection withFIGS. 8A-8B and will be re-described herein. FIG. 8A is a graphillustrating exemplary MIMO communication signal waveforms transmittedby MIMO transmitter antennas 156(2)(1), 156(2)(2) in a remote unit 44(1)without amplitude adjustment. As shown in FIG. 8A, when amplitudeadjustment is not provided to the MIMO transmitter antennas 156(2)(1),156(2)(2) in FIG. 6A, the downlink MIMO communications signals 82D(3),82D(4) both have the same first amplitudes x. FIG. 8B is a graphillustrating exemplary MIMO communication signal waveforms transmittedby MIMO transmitter antennas 156(2)(1), 156(2)(2) in a remote unit 44(1)when amplitude adjustment is provided to the MIMO transmitter antenna156(2)(2). As can be seen in FIG. 8B, a second amplitude y of thedownlink MIMO communications signal 82D(4) is smaller than the firstamplitude x of the downlink MIMO communications signal 82D(3). Anamplitude adjustment factor α is computed as the ratio between y and x(α=y/x). For example, the amplitude adjustment factor α=0.7 indicatesthat the second amplitude y is 70% of the first amplitude x. Aspreviously described in FIGS. 6A-6C, such amplitude difference, inconjunction with different polarization states 158(1), 158(2), can helpreduce or eliminate periodic destructive interferences between thedownlink MIMO communications signals 82D(3), 82D(4) at the MIMOreceivers 162. Note that although in FIG. 8B, the amplitude y of thedownlink MIMO communications signal 82D(4) is shown to be smaller thanthe amplitude x of the downlink MIMO communications signal 83D(3), it ispossible to amplify the amplitude y of the downlink MIMO communicationssignal 82D(4) to be larger than the amplitude x of the downlink MIMOcommunications signal 82D(3). Further, although the downlink MIMOcommunications signals 82D(4) in FIGS. 8A and 8B are shown to have thesame phase, it is also possible to simultaneously phase shift andamplitude adjust the downlink MIMO communications signal 82D(4).

To illustrate the performance improvements provided by the amplitudeadjustment circuits 120(1)-120(3) in the MIMO distributed antennasystems 40(1)-40(3) in FIGS. 6A-6C, FIG. 8C illustrates a graph 180illustrating exemplary performance degradation curves for a givenplacement distance between the MIMO transmitters 154(1), 154(2). Similarto graph 102 in FIG. 3B, FIG. 8C illustrates MIMO distributed antennasystems 40(1)-40(3) capacity on the y-axis in units of Gigabits persecond (Gbps) versus MIMO transmitter antennas 78(1), 78(2) separationdistance on the x-axis in units of centimeters (cm). The capacitydegradation curve 182 in FIG. 8C, which is equivalent to the capacitycurve 104 in FIG. 3B, illustrates severe periodic capacity dipsresulting from periodic destructive interference when amplitudeadjustment techniques described above for MIMO distributed antennasystems 40(1)-40(3) are not employed. As shown in the graph 180, for agiven transmitter antenna separation distance and a given wirelessdistance (e.g., a distance between a wireless transmitter and a wirelessreceiver), a capacity degradation curve 184 and a capacity degradationcurve 186 illustrate different degrees of capacity degradations whenamplitude adjustment techniques described above for MIMO distributedantenna systems 40(1)-40(3) are employed. In this non-limiting example,the capacity degradation curves 184 and 186 are associated withamplitude adjustment factors α=0.7 and α=0.9, respectively. As can beseen in the capacity degradation curves 184, 186, periodic capacitydips, although not completely eliminated, do become more moderate asresult of reduced periodic destructive interference provided byamplitude adjustment techniques described above for MIMO distributedantenna systems 40(1)-40(3).

FIG. 8D is a graph 190 illustrating an exemplary effective antennacoverage versus placement distance between MIMO transmitters 154(1),154(2) in the distributed antenna systems 40(1)-40(3) in FIGS. 6A-6C,for a two (2) cm placement distance between the MIMO receivers 162(1),162(2). When the amplitude adjustment techniques described above for theMIMO distributed antenna systems 40(1)-40(3) are employed, coveragecurve 192 illustrates consistent 100% antenna coverage regardless ofplacement distance between MIMO transmitters 154(1), 154(2) in thedistributed antenna systems 40(1)-40(3) in FIGS. 6A-6C. When theamplitude adjustment techniques described above for the MIMO distributedantenna systems 40(1)-40(3) are not employed, coverage curve 194illustrates inconsistent antenna coverage dependent upon placementdistance between MIMO transmitters 154(1), 154(2) in the distributedantenna systems 40(1)-40(3) in FIGS. 6A-6C.

It may also be desired to provide high-speed wireless digital dataservice connectivity with remote units in the MIMO distributed antennasystems disclosed herein. One example would be WiFi. WiFi was initiallylimited in data rate transfer to 12.24 Mb/s and is now provided at datatransfer rates of up to 54 Mb/s using WLAN frequencies of 2.4 GHz and5.8 GHz. While interesting for many applications, WiFi has proven tohave too small a bandwidth to support real time downloading ofuncompressed high definition (HD) television signals to wireless clientdevices. To increase data transfer rates, the frequency of wirelesssignals could be increased to provide larger channel bandwidth. Forexample, an extremely high frequency in the range of 30 GHz to 300 GHzcould be employed. For example, the sixty (60) GHz spectrum is an EHFthat is an unlicensed spectrum by the Federal Communications Commission(FCC) and that could be employed to provide for larger channelbandwidths. However, high frequency wireless signals are more easilyattenuated or blocked from traveling through walls or other buildingstructures where distributed antenna systems are installed.

Thus, the embodiments disclosed herein can include distribution ofextremely high frequency (EHF) (i.e., approximately 30-approximately 300GHz), as a non-limiting example. The MIMO distributed antenna systemsdisclosed herein can also support provision of digital data services towireless clients. The use of the EHF band allows for the use of channelshaving a higher bandwidth, which in turn allows more data intensivesignals, such as uncompressed HD video to be communicated withoutsubstantial degradation to the quality of the video. As a non-limitingexample, the distributed antenna systems disclosed herein may operate atapproximately sixty (60) GHz with approximately seven (7) GHz bandwidthchannels to provide greater bandwidth to digital data services. Thedistributed antenna systems disclosed herein may be well suited to bedeployed in an indoor building or other facility for delivering ofdigital data services.

It may be desirable to provide MIMO distributed antenna systems,according to the embodiments disclosed herein, that provide digital dataservices for client devices. For example, it may be desirable to providedigital data services to client devices located within a distributedantenna system. Wired and wireless devices may be located in thebuilding infrastructures that are configured to access digital dataservices. Examples of digital data services include, but are not limitedto, Ethernet, WLAN, WiMax, WiFi, DSL, and LTE, etc. Ethernet standardscould be supported, including but not limited to, 100 Mb/s (i.e., fastEthernet) or Gigabit (Gb) Ethernet, or ten Gigabit (10G) Ethernet.Examples of digital data services include, but are not limited to, wiredand wireless servers, wireless access points (WAPs), gateways, desktopcomputers, hubs, switches, remote radio heads (RRHs), baseband units(BBUs), and femtocells. A separate digital data services network can beprovided to provide digital data services to digital data devices.

FIG. 9 is a schematic diagram representation of additional detailillustrating components that could be employed in any of the componentsor devices disclosed herein, but only if adapted to execute instructionsfrom an exemplary computer-readable medium to perform any of thefunctions or processing described herein. In this regard, such componentor device may include a computer system 220 within which a set ofinstructions for performing any one or more of the location servicesdiscussed herein may be executed. The computer system 220 may beconnected (e.g., networked) to other machines in a LAN, an intranet, anextranet, or the Internet. While only a single device is illustrated,the term “device” shall also be taken to include any collection ofdevices that individually or jointly execute a set (or multiple sets) ofinstructions to perform any one or more of the methodologies discussedherein. The computer system 220 may be a circuit or circuits included inan electronic board card, such as, a printed circuit board (PCB), aserver, a personal computer, a desktop computer, a laptop computer, apersonal digital assistant (PDA), a computing pad, a mobile device, orany other device, and may represent, for example, a server or a user'scomputer.

The exemplary computer system 220 in this embodiment includes aprocessing device or processor 222, a main memory 224 (e.g., read-onlymemory (ROM), flash memory, dynamic random access memory (DRAM), such assynchronous DRAM (SDRAM), etc.), and a static memory 226 (e.g., flashmemory, static random access memory (SRAM), etc.), which may communicatewith each other via a data bus 228. Alternatively, the processing device222 may be connected to the main memory 224 and/or static memory 226directly or via some other connectivity means. The processing device 222may be a controller, and the main memory 224 or static memory 226 may beany type of memory.

The processing device 222 represents one or more general-purposeprocessing devices, such as a microprocessor, central processing unit,or the like. More particularly, the processing device 222 may be acomplex instruction set computing (CISC) microprocessor, a reducedinstruction set computing (RISC) microprocessor, a very long instructionword (VLIW) microprocessor, a processor implementing other instructionsets, or other processors implementing a combination of instructionsets. The processing device 222 is configured to execute processinglogic in instructions 230 for performing the operations and stepsdiscussed herein.

The computer system 220 may further include a network interface device232. The computer system 220 also may or may not include an input 234,configured to receive input and selections to be communicated to thecomputer system 220 when executing instructions. The computer system 220also may or may not include an output 236, including but not limited toa display, a video display unit (e.g., a liquid crystal display (LCD) ora cathode ray tube (CRT)), an alphanumeric input device (e.g., akeyboard), and/or a cursor control device (e.g., a mouse).

The computer system 220 may or may not include a data storage devicethat includes instructions 238 stored in a computer-readable medium 240.The instructions 238 may also reside, completely or at least partially,within the main memory 224 and/or within the processing device 222during execution thereof by the computer system 220, the main memory 224and the processing device 222 also constituting computer-readablemedium. The instructions 238 may further be transmitted or received overa network 242 via the network interface device 232.

While the computer-readable medium 240 is shown in an exemplaryembodiment to be a single medium, the term “computer-readable medium”should be taken to include a single medium or multiple media (e.g., acentralized or distributed database, and/or associated caches andservers) that store the one or more sets of instructions. The term“computer-readable medium” shall also be taken to include any mediumthat is capable of storing, encoding, or carrying a set of instructionsfor execution by the processing device and that cause the processingdevice to perform any one or more of the methodologies of theembodiments disclosed herein. The term “computer-readable medium” shallaccordingly be taken to include, but not be limited to, solid-statememories, optical and magnetic medium, and carrier wave signals.

The embodiments disclosed herein include various steps. The steps of theembodiments disclosed herein may be formed by hardware components or maybe embodied in machine-executable instructions, which may be used tocause a general-purpose or special-purpose processor programmed with theinstructions to perform the steps. Alternatively, the steps may beperformed by a combination of hardware and software.

The embodiments disclosed herein may be provided as a computer programproduct, or software, that may include a machine-readable medium (orcomputer-readable medium) having stored thereon instructions, which maybe used to program a computer system (or other electronic devices) toperform a process according to the embodiments disclosed herein. Amachine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For example, a machine-readable medium includes: amachine-readable storage medium (e.g., ROM, random access memory(“RAM”), a magnetic disk storage medium, an optical storage medium,flash memory devices, etc.); a machine-readable transmission medium(electrical, optical, acoustical, or other form of propagated signals(e.g., carrier waves, infrared signals, digital signals, etc.)); and thelike.

Unless specifically stated otherwise and as apparent from the previousdiscussion, it is appreciated that throughout the description,discussions utilizing terms such as “processing,” “computing,”“determining,” “displaying,” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data and memories represented asphysical (electronic) quantities within the computer system's registersinto other data similarly represented as physical quantities within thecomputer system memories or registers or other such information storage,transmission, or display devices.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various systems may beused with programs in accordance with the teachings herein, or it mayprove convenient to construct more specialized apparatuses to performthe required method steps. The required structure for a variety of thesesystems will appear from the description above. In addition, theembodiments described herein are not described with reference to anyparticular programming language. It will be appreciated that a varietyof programming languages may be used to implement the teachings of theembodiments as described herein.

Those of skill in the art will further appreciate that the variousillustrative logical blocks, modules, circuits, and algorithms describedin connection with the embodiments disclosed herein may be implementedas electronic hardware, instructions stored in memory or in anothercomputer-readable medium and executed by a processor or other processingdevice, or combinations of both. The components of the distributedantenna systems described herein may be employed in any circuit,hardware component, integrated circuit (IC), or IC chip, as examples.Memory disclosed herein may be any type and size of memory and may beconfigured to store any type of information desired. To clearlyillustrate this interchangeability, various illustrative components,blocks, modules, circuits, and steps have been described above generallyin terms of their functionality. How such functionality is implementeddepends on the particular application, design choices, and/or designconstraints imposed on the overall system. Skilled artisans mayimplement the described functionality in varying ways for eachparticular application, but such implementation decisions should not beinterpreted as causing a departure from the scope of the presentembodiments.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a processor, a Digital Signal Processor (DSP), anApplication Specific Integrated Circuit (ASIC), a Field ProgrammableGate Array (FPGA), or other programmable logic device, a discrete gateor transistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Furthermore,a controller may be a processor. A processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices (e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration).

The embodiments disclosed herein may be embodied in hardware and ininstructions that are stored in hardware, and may reside, for example,in RAM, flash memory, ROM, Electrically Programmable ROM (EPROM),Electrically Erasable Programmable ROM (EEPROM), registers, a hard disk,a removable disk, a CD-ROM, or any other form of computer-readablemedium known in the art. An exemplary storage medium is coupled to theprocessor such that the processor can read information from, and writeinformation to, the storage medium. In the alternative, the storagemedium may be integral to the processor. The processor and the storagemedium may reside in an ASIC. The ASIC may reside in a remote station.In the alternative, the processor and the storage medium may reside asdiscrete components in a remote station, base station, or server.

It is also noted that the operational steps described in any of theexemplary embodiments herein are described to provide examples anddiscussion. The operations described may be performed in numerousdifferent sequences other than the illustrated sequences. Furthermore,operations described in a single operational step may actually beperformed in a number of different steps. Additionally, one or moreoperational steps discussed in the exemplary embodiments may becombined. Those of skill in the art will also understand thatinformation and signals may be represented using any of a variety oftechnologies and techniques. For example, data, instructions, commands,information, signals, bits, symbols, and chips, that may be referencesthroughout the above description, may be represented by voltages,currents, electromagnetic waves, magnetic fields, or particles, opticalfields or particles, or any combination thereof.

Further and as used herein, it is intended that terms “fiber opticcables” and/or “optical fibers” include all types of single mode andmulti-mode light waveguides, including one or more optical fibers thatmay be upcoated, colored, buffered, ribbonized, and/or have otherorganizing or protective structure in a cable such as one or more tubes,strength members, jackets, or the like. The optical fibers disclosedherein can be single mode or multi-mode fibers. Likewise, other types ofsuitable optical fibers include bend-insensitive optical fibers, or anyother expedient of a medium for transmitting light signals. An exampleof a bend-insensitive, or bend resistant, optical fiber is ClearCurve®Multimode fiber, commercially available from Corning Incorporated.Suitable fibers of this type are disclosed, for example, in U.S. PatentApplication Publication Nos. 2008/0166094 and 2009/0169163, thedisclosures of which are incorporated herein by reference in theirentireties.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Since modifications combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A distributed communication system, comprising: acentral unit; a plurality of optical fibers; and a plurality ofmultiple-input, multiple-output (MIMO) remote units coupled to thecentral unit by the plurality of optical fibers, at least one of theMIMO remote units being configured to wirelessly distribute MIMOcommunications signals to wireless client devices located in a coveragearea, the at least one MIMO remote unit comprising: a first MIMOtransmitter comprising a first MIMO transmitter antenna configured totransmit MIMO communications signals in a first polarization and asecond MIMO transmitter antenna configured to transmit MIMOcommunications signals in a second polarization different from the firstpolarization; and a second MIMO transmitter comprising a third MIMOtransmitter antenna configured to transmit MIMO communications signalsin the first polarization and a fourth MIMO transmitter antennaconfigured to transmit MIMO communications signals in the secondpolarization, wherein the first MIMO transmitter is configured to:receive a first optical downlink MIMO communications signal at a firstamplitude over a first one of the optical fibers and, to transmit thefirst downlink MIMO communications signal as a first electrical downlinkMIMO communications signal wirelessly over the first MIMO transmitterantenna in the first polarization; and receive a second optical downlinkMIMO communications signal at the first amplitude and, to transmit thesecond downlink MIMO communications signal as a second electricaldownlink MIMO communications signal wirelessly over the second MIMOtransmitter antenna in the second polarization, and wherein the secondMIMO transmitter is configured to: receive a third optical downlink MIMOcommunications signal at the first amplitude and to transmit the thirddownlink MIMO communications signal as a third electrical downlink MIMOcommunications signal wirelessly over the third MIMO transmitter antennain the first polarization; and receive a fourth optical downlink MIMOcommunications signal and, to transmit the fourth downlink MIMOcommunications signal at a second amplitude modified from the firstamplitude, as a fourth electrical downlink MIMO communications signalwirelessly over the fourth MIMO transmitter antenna in the secondpolarization.
 2. The distributed communication system of claim 1,wherein the at least one MIMO remote unit comprises at least oneoptical-to-electrical (O/E) converter configured to convert the firstoptical downlink MIMO communications signal before transmission of thefirst electrical downlink MIMO communications signal.
 3. The distributedcommunication system of claim 2, further comprising at least oneamplitude adjustment circuit configured to amplitude adjust the fourthoptical downlink MIMO communications signal at the second amplitude. 4.The distributed communication system of claim 3, wherein the at leastone amplitude adjustment circuit is further configured to beelectronically tunable to adjust the fourth optical downlink MIMOcommunications signal at the second amplitude.
 5. The distributedcommunication system of claim 3, wherein the amplitude adjustmentcircuit is configured to attenuate the fourth optical downlink MIMOcommunications signal at the second amplitude.
 6. The distributedcommunication system of claim 3, wherein the amplitude adjustmentcircuit is configured to amplify the fourth optical downlink MIMOcommunications signal at the second amplitude.
 7. The distributedcommunication system of claim 3, wherein the amplitude adjustmentcircuit is configured to adjust a bias signal of an optical modulator toamplitude adjust the fourth optical downlink MIMO communications signalat the second amplitude.
 8. The distributed communication system ofclaim 3, wherein the second MIMO transmitter is configured to receivethe fourth optical downlink MIMO communications signal in the secondamplitude as a result of an amplitude adjustment of the fourth opticaldownlink MIMO communications signal.
 9. The distributed communicationsystem of claim 3, wherein at least one of the first electrical downlinkMIMO communications signal, the second electrical downlink MIMOcommunications signal, the third electrical downlink MIMO communicationssignal, and the fourth electrical downlink MIMO communications signalinclude a carrier frequency having an extremely high frequency (EHF)between approximately 30 GHz and approximately 300 GHz.
 10. Thedistributed communication system of claim 3, wherein: the first MIMOtransmitter is further configured to transmit the first downlink MIMOcommunications signal and second downlink MIMO communications signalwirelessly to a line-of-sight (LOS) wireless client; and the second MIMOtransmitter is configured to transmit the third MIMO communicationssignal and fourth downlink MIMO communications signal wirelessly to aline-of-sight (LOS) wireless client.
 11. The distributed communicationsystem of claim 2, wherein the second MIMO transmitter is configured toreceive the fourth optical downlink MIMO communications signal at thesecond amplitude as a result of an amplitude adjustment of the fourthoptical downlink MIMO communications signal in the central unit.
 12. Amethod of transmitting multiple-input, multiple-output (MIMO)communications signals to wireless client devices in a coverage area ofa distributed communication system, comprising: receiving a firstoptical downlink MIMO communications signal at a first amplitude over afirst downlink communications medium; converting the first opticaldownlink MIMO communications signal to a first electrical downlink MIMOcommunications signal; transmitting the first electrical downlink MIMOcommunications signal over a first MIMO transmitter antenna in a firstpolarization; receiving a second optical downlink MIMO communicationssignal at the first amplitude; converting the second optical downlinkMIMO communications signal to a second electrical downlink MIMOcommunications signal; transmitting the second electrical downlink MIMOcommunications signal over a second MIMO transmitter antenna in a secondpolarization; receiving a third optical downlink MIMO communicationssignal at the first amplitude; converting the third optical downlinkMIMO communications signal to a third electrical downlink MIMOcommunications signal; transmitting the third electrical downlink MIMOcommunications signal over a third MIMO transmitter antenna in the firstpolarization; receiving a fourth optical downlink MIMO communicationssignal; converting the fourth optical downlink MIMO communicationssignal to a fourth electrical downlink MIMO communications signal; andtransmitting the fourth electrical downlink MIMO communications signalat a second amplitude modified from the first amplitude, over a fourthMIMO transmitter antenna in the second polarization.
 13. The method ofclaim 12, wherein: transmitting the first electrical downlink MIMOcommunications signal comprises transmitting to a line-of-sight (LOS)wireless client; transmitting the second electrical downlink MIMOcommunications signal comprises transmitting to a LOS wireless client;transmitting the third electrical downlink MIMO communications signalcomprises transmitting to a LOS wireless client; and transmitting thefourth electrical downlink MIMO communications signal comprisestransmitting to a LOS wireless client.
 14. The method of claim 12,further comprising amplitude adjusting the fourth optical downlink MIMOcommunications signal at the second amplitude.
 15. The method of claim14, further comprising receiving the fourth optical downlink MIMOcommunications signal at the second amplitude modified from the firstamplitude in a central unit.
 16. The method of claim 14, furthercomprising receiving the fourth optical downlink MIMO communicationssignal at the second amplitude modified from the first amplitude in thefourth downlink communications medium.
 17. The method of claim 14,comprising amplitude adjusting the fourth downlink MIMO communicationssignal at the second amplitude.